1. Field of the Invention
This invention relates to a transistor, particularly to a structure of a high frequency and high output power transistor where a plurality of transistor units are connected in parallel.
2. Description of the Prior Art
As the existing high frequency and high output power transistor where a plurality of transistor units for low level signals on a semiconductor substrate are connected in parallel, the overlay type transistor (e.g. U.S. Pat. No. 3,434,091), the ring-emitter type transistor (e.g. U.S. Pat. No. 4,157,561) etc. have been developed and already put to practical use.
In these transistors, the base (lead out) electrode and the emitter (lead out) electrode which connect individual transistors formed in isolation from each other on an insulation film such as silicon dioxide (SiO.sub.2) are formed on said semiconductor substrate and disposed in the form of the teeth of a comb on each transistor unit.
Therefore, if said insulation film has a pinhole or is partially missing, the base and emitter are likely shorted. When the width of said electrode or the number of electrodes increases in order to increase power, the probability of shorting is naturally increased.
Such short-circuiting of the base and emitter deteriorates the yield of manufacture and the reliability of the resultant transistor.
On the other hand, the individual transistor units formed on the semiconductor substrate which structures these transistors do not always have the same electric characteristics, and when said transistor operates, a particular transistor unit may suffer secondary breakdown due to an excessive load resulting from irregularity of the characteristics of each transistor unit. Therefore, a transistor of this type includes a stabilizing resistor in the emitter circuit of each transistor unit in order to equalize a load applied on the transistor units by utilizing a negative feedback effect by such stabilizing resistor.
Various means are already proposed for insetting said stabilizing resistor.
In one method proposed as the stabilizing resistor material, a metallic substance such as nichrome (Ni-Cr) etc. or a polycrystalline semiconductor substance such as polycrystalline silicon is used. Such resistor materials are deposited and laminated in the electrode window provided in the insulating film covering the emitter regions and the electrode wiring is connected to one end, or a resistor material is formed on said insulating film and connected at a central portion thereof to the electrode wiring through an electrode window provided on said insulating film covering the emitter regions.
In another method, a diffusion region with the same conductivity type as the emitter region is formed in the base region simultaneously with said emitter region or in a different process from that for said emitter region for use as a stabilizing resistor. Said diffusion resistor region is formed in such a way as to contact the emitter region at one end with the other end connected to the electrode wiring; or said diffusion resistor region is isolated by being formed at a distance from the corresponding emitter region so that one end is connected to the emitter region with the electrode wiring and the other end is let to the electrode wiring.
However such existing general methods have the disadvantages that manufacturing processes increase, desired electrical characteristics cannot be obtained and moreover high integration density cannot be obtained.
For example, in the case of a metallic substance or a polycrystalline semiconductor substance being used as a resistor material, the processes for depositing these materials and for forming them into the desired pattern inevitably add to the manufacturing process. In addition, such attempts to increase the number of emitters for obtaining higher output power makes patterning of the resistor material difficult, resulting in lower integration density and increasing the physical dimensions of a semiconductor element (chip size).
Moreover, there is difficulty in the selection of a material which simultaneously satisfies the ohmic value and current capacity.
In the case that said diffusion region, having an impurity of the same conductivity type as the emitter region, is diffused into the base region to be used as a stabilizing resistor, carrier injection occurs from said diffusion resistance region to the base region during operation of a transistor and thereby the current amplification factor (h.sub.FE) of said transistor is reduced. Further, an attempt to form the emitter region and said diffused resistor region in different diffusion processes will naturally cause an increase in the number of manufacturing steps.